On an express train bound for Peking, Doctor Michael Bachlin, Joseph Cotten, travels from Shanghai to recover a missing shipment of medical supplies for the United Nations’ World Health Organization, accompanied by a small, tense ensemble. Also aboard are Father Joseph Murray, a steadying presence, and Kwon, a charismatic but opaque businessman with his own shadowy business interests. Just before departure, Kwon’s son, Ti Shen, Robert W. Lee, attempts to board, but his mother, Li Eiu, has him arrested by Nationalist agents, and Li Eiu herself soon boards the train.
Another passenger is Danielle Grenier, Corinne Calvet, a nightclub singer and former lover Michael met in Paris. Kwon invites Danielle and Michael to the dining car, and tensions simmer as Father Murray engages in a clash with Wong, Benson Fong, a reporter with ardent Communist views who dislikes Michael. Danielle explains that after their split, she remarried, but her husband died a year later.
In Danielle’s compartment, Li Eiu is found beaten; the next day, when the train stops to pick up soldiers, Kwon passes a message to a vendor. Michael hopes to rekindle romance, but Danielle hesitates, noting she has been involved with too many others. Li Eiu is discovered with a knife wound, and while Kwon claims it was a suicide attempt, Michael grows suspicious.
A rail blockade strands the train, and counter-revolutionaries attack as soldiers on board surrender and are swiftly executed. Michael, Danielle, Father Murray, and Wong learn that Kwon is the attackers’ ringleader, and the group is driven to a nearby farm hideout. Kwon, who once followed the Communist cause, now traffics in the black market, including the stolen medical supplies Michael seeks to recover. He forces Michael to contact Peking, offering to release the train and its passengers in exchange for Ti Shen’s freedom.
Michael arranges to have Ti Shen flown to the farm hideout, while Danielle reveals she once spied for a cause and that her late husband was a Communist. The hostage-release plan hinges on Peking delivering Ti Shen without alerting the aircraft escort. When Wong confronts Kwon about his treachery, the reporter is tortured as his hands are burned with a poker. After Ti Shen arrives, the pilot radios Peking that dawn’s departure will begin with Michael, but Kwon shoots the pilot, breaking the deal.
Michael tends Wong’s wounds and explains Li Eiu’s plea to protect her son from her husband’s influence. Li Eiu then confronts Kwon, stabbing him before dying, and she begs Ti Shen to aid Michael and the others in escaping. Under Father Murray’s guard, Michael escorts Ti Shen back toward safety and eventually finds Danielle, who has returned to the farmhouse. Michael binds Ti Shen, returns to Kwon’s base, eliminates two guards, and abducts Danielle.
Back at the train, Wong hops into a jeep with two soldiers and hurls a grenade, wounding Father Murray. Michael returns fire with a machine gun, killing the attackers. As the train gains speed, more pursuers close in, but Ti Shen helps by firing on the soldiers before being shot and killed; before dying, he reveals the location of the stolen medicine cache. In the end, Michael, Danielle, and Father Murray manage to escape, their chances slim but persistent, as the mission to retrieve the medical supplies reaches a perilous conclusion.
